Recently the development of finders suitable for cameras has flourished according to market demand for miniaturization of cameras. Real-image-type finders have become most prevalent due to their being compact. Moreover, the development of high magnification zoom finders suitable for use with high magnification cameras has increased.
Compact real-image-type finders with a small number of lens elements have generally been formed using an objective lens system and an eye lens system with an intermediate erecting system in which a condenser lens is bonded with adhesive to two triangular prisms that are joined to form a Porro prism. Recently, with the progress of plastic molding techniques, the cost of making real-image-type finders having an extremely small number of lens elements has been considerably reduced. These finders generally include an erecting eyepiece system and an objective lens system. The erecting eyepiece system may be formed using a condenser lens, a Porro prism, and an eye lens that are each made of plastic and molded as an integral unit. Such finders have been disclosed in Japanese Laid Open Patent Applications H4-19705 and H11-109446. These finders are practicable because the objective optical system is relatively small and may be easily assembled when the lens has a fixed focus or zooms with a zoom ratio of about 2.
However, using the above-mentioned, conventional design results in large-size finders when the objective lens system is required to have a higher zoom ratio. Thus, such a design cannot simultaneously satisfy the market demand for miniaturization and higher zoom ratios. Even though the objective lens system may have a fixed focus or a low magnification in these well-known examples, the erecting eyepiece system has a relatively long focal length. This allows sufficient space along the optical path for placement of the four reflecting surfaces needed in the erecting eyepiece system in order to invert and erect the image. However, the erecting eyepiece system prevents such a design from being compact. Further, for a wide-angle image field, the image size increases. Thus, the finder becomes large-sized.
In order to achieve a high magnification with a real-image-type zoom finder that is compact, the objective lens system must achieve a high zoom ratio. This causes the minium number of lens elements in the objective lens system to increase in order to maintain high image quality, and also causes the overall length of the objective lens system to increase. In the above-discussed, conventional lens systems, the objective lens becomes large-sized due to its required high magnification.
Shortening the focal length range of the objective lens system is also a method for seeking both high magnification and compactness in a real-image-type zoom finder. However, this approach results in the image size formed by the objective lens system decreasing and the zoom ratio decreasing, or the optical path length of the erecting system must be decreased. Thus, either the finder must have less zoom capability or the optical path length of the erecting system must be shorter. Even if the focal length of the eye lens system can be decreased in order to maintain a high zoom ratio, such a reduction in focal length of the eve lens system is undesirable as it requires the erecting optical system to be divided into two parts, one part before and one part after the imaging position of the objective lens system.
Thus, there is market demand for a high magnification, real-image-type zoom finder that is compact, uses a small number of lens elements, is inexpensive, easy to assemble, and provides a satisfactory image to a viewer using the finder.
This invention relates to a real-image-type zoom finder suitable for conventional film cameras or electronic cameras, and particularly to a real-image-type zoom finder having a high zoom ratio. The object of the present invention is to miniaturize the objective optical system as well as to miniaturize the erecting eyepiece lens system of a finder in order to obtain a zoom finder that has a high zoom ratio and is compact. More particularly, the object of the invention, is to provide a real-image-type finder having a zoom ratio of about four that provides high quality imaging, is compact, and is formed of a small number of lens elements.